This invention relates to a glazing pane, more especially intended for automobile vehicles, and notably for equipping the side regions with sliding panes. More specifically, the invention concerns a safety glass pane of the type known as laminated, of which the glass sheet or sheets have undergone a thermal treatment and are assembled to one another by a sheet of plastics material. The invention also has as its subject a method for the production of such a pane.
For improving the mechanical strength of a glass sheet, it is well known that it is advantageous to carry out a treatment known as thermal toughening enabling the surface of the glass to be brought into compression, the compressive strength of glass being much higher than its tensile strength.
Thermal toughening consists of cooling the surface of a glass sheet in such a way as to solidify it at the surface before the interior becomes fixed, with the result that, when cooling continues throughout the thickness, the interior cannot contract normally, since it is prevented by the solidification of the surface. The interior is then subject to tensile stresses, whereas at the surface compressive stresses form.
When the state of the stresses induced in a glass sheet by such a thermal treatment is considered, three principal regions should be distinguished. The first region is the central region, little influenced by the edge and where, to a first approximation, it may be assumed that the glass sheet behaves as a glass sheet of infinite dimensions. If this glass sheet is of constant thickness and is cooled uniformly on both its faces, the stresses are distributed through the thickness of the sheet in accordance with a parabolic distribution, characterized by a surface compressive stress and a tensile stress in the median plane of the sheet, also termed core tensile stress. This distribution, known as the thickness stress distribution, is isotropic, the integral of the stresses along the thickness being zero, as also is the integral of the moments.
In theory, the value of the surface compressive stress is exactly equal to twice the value of the core tensile stress, although in practice a higher value should be adopted.
Moreover, a glass sheet certainly has finite dimensions and at its arrises the cooling also affects the edge face and locally concerns the entire thickness of the glass sheet. In the region bordering on the edges thus affected, the integral of the stresses through the thickness is no longer zero but leads to a preponderance of compressive stresses. In this region, the thickness stresses are therefore anisotropic. The stress tangential to the surface, averaged over the entire thickness of the glass sheet, is termed the edge stress. This marginal region has a quite special importance, notably during installation and in the case of panes mounted with a free edge, such as for example the sliding lateral panes of an automobile vehicle. The width of this marginal region is generally of the order of 2 to 3 times the thickness of the glass sheet.
However, these edge stresses are balanced locally by tensile stresses. At the frontier between the marginal region of the edge stresses and the central region, there thus exists an intermediate region where the integral of the difference of the stresses is not zero and leads to a preponderance of tensile stresses. This intermediate region, therefore having anisotropic stresses, can reach a width of several centimeters and is a very brittle region because the glass there is locally prestressed in tension. The higher the edge stresses, the higher these tensile stresses will be.
It is important to emphasize that the foregoing analysis is true even in the case of non-toughened glass sheets which, in contrast, are annealed as are, in particular, the glass sheets more especially intended for the manufacture of laminated panes. In accordance with the technique most commonly used today these sheets of glass, after they have been cut to the desired shape, are superimposed in pairs on a curving mold, such as an annular frame open at its center, and are reheated on this mold in a furnace for curving by gravity. When the desired shape has been obtained, the assembly is moved into an annealing station for a slow cooling of the glass, thus enabling the internal stresses induced during the curving to be relaxed. But this annealing does not prevent a more pronounced cooling of the edges, which are cooled on three surfaces and, for this reason, edge stresses are certainly observed in the marginal region, compensated by tensile stresses in a frontier region.
In any case, an attempt is always made to incorporate edge stresses, in order to increase the mechanical strength of the edges. This is why treatments may even be carried out of the light cooling blowing type on the edges, so as to increase the edge stresses even although the compressive stress at the surface does not generally exceed 5-10 MPa for the central region. In this case, however, the level of the tensile stress in the frontier zone is thus increased, and for this reason this zone becomes still more fragile.
For glazing panes such as windscreens, the problem posed by this frontier zone and/or the low level of the edge stresses may be eliminated by the use of mounting frames glued or molded around the pane, which serve as protection. But for a pane mounted flush or with a free edge, no protection is possible and the pane has a small, particularly fragile region, which is notably sensitive to flying stone chippings.
On the other hand, it has been indicated above that thermal toughening enables a higher mechanical strength to be conferred upon a pane. If, however, the pane is successfully broken, it shatters into a multitude of fragments, through which visibility is virtually zero. In contrast, in the case of a laminated body of annealed glass, the gravel impact causes the formation of a star with a small number of splinters around the point of impact and a visibility through the pane which is very largely preserved. It must be understood, however, that this is achieved at the expense of a much lower resistance to damage from gravel than in the case of a toughened pane. Therefore, although breakages are less annoying, they are also much more frequent.
In these circumstances it has been proposed to create laminated automobile glazing panes of toughened glass, but with the major disadvantage of a pane that is entirely unusable if impact succeeds in breaking it. Furthermore, such panes are very difficult to produce in that, in order to avoid excessive weight, the laminated panes for automobiles are generally composed of thin glass sheets which, by reason of their thinness, present some problems of reproducibility of the curvature, but in particular which are virtually impossible to toughen to toughening values, and therefore notably to surface compressive stress values, of a level equivalent to that adopted for the toughening of conventional monolithic glasses, such as the lateral window panes or rear windows. In any case, this solution in no way resolves the problem of the zone subjected to tension by the edge stresses.